7.8. Using MEncoder
to create VCD/SVCD/DVD-compliant files

7.8.1. Format Constraints

MEncoder is capable of creating VCD, SCVD
and DVD format MPEG files using the
libavcodec library.
These files can then be used in conjunction with
vcdimager
or
dvdauthor
to create discs that will play on a standard set-top player.

The DVD, SVCD, and VCD formats are subject to heavy constraints.
Only a small selection of encoded picture sizes and aspect ratios are
available.
If your movie does not already meet these requirements, you may have
to scale, crop or add black borders to the picture to make it
compliant.

[a]
These resolutions are rarely used for DVDs because
they are fairly low quality.

If your movie has 2.35:1 aspect (most recent action movies), you will
have to add black borders or crop the movie down to 16:9 to make a DVD or VCD.
If you add black borders, try to align them at 16-pixel boundaries in
order to minimize the impact on encoding performance.
Thankfully DVD has sufficiently excessive bitrate that you do not have
to worry too much about encoding efficiency, but SVCD and VCD are
highly bitrate-starved and require effort to obtain acceptable quality.

7.8.1.2. GOP Size Constraints

DVD, VCD, and SVCD also constrain you to relatively low
GOP (Group of Pictures) sizes.
For 30 fps material the largest allowed GOP size is 18.
For 25 or 24 fps, the maximum is 15.
The GOP size is set using the keyint option.

7.8.1.3. Bitrate Constraints

VCD video is required to be CBR at 1152 kbps.
This highly limiting constraint also comes along with an extremely low vbv
buffer size of 327 kilobits.
SVCD allows varying video bitrates up to 2500 kbps, and a somewhat less
restrictive vbv buffer size of 917 kilobits is allowed.
DVD video bitrates may range anywhere up to 9800 kbps (though typical
bitrates are about half that), and the vbv buffer size is 1835 kilobits.

7.8.2. Output Options

MEncoder has options to control the output
format.
Using these options we can instruct it to create the correct type of
file.

The options for VCD and SVCD are called xvcd and xsvcd, because they
are extended formats.
They are not strictly compliant, mainly because the output does not
contain scan offsets.
If you need to generate an SVCD image, you should pass the output file to
vcdimager.

7.8.2.1. Aspect Ratio

The aspect argument of -lavcopts is used to encode
the aspect ratio of the file.
During playback the aspect ratio is used to restore the video to the
correct size.

16:9 or "Widescreen"

-lavcopts aspect=16/9

4:3 or "Fullscreen"

-lavcopts aspect=4/3

2.35:1 or "Cinemascope" NTSC

-vf scale=720:368,expand=720:480 -lavcopts aspect=16/9

To calculate the correct scaling size, use the expanded NTSC width of
854/2.35 = 368

2.35:1 or "Cinemascope" PAL

-vf scale=720:432,expand=720:576 -lavcopts aspect=16/9

To calculate the correct scaling size, use the expanded PAL width of
1024/2.35 = 432

7.8.2.2. Maintaining A/V sync

In order to maintain audio/video synchronization throughout the encode,
MEncoder has to drop or duplicate frames.
This works rather well when muxing into an AVI file, but is almost
guaranteed to fail to maintain A/V sync with other muxers such as MPEG.
This is why it is necessary to append the
harddup video filter at the end of the filter chain
to avoid this kind of problem.
You can find more technical information about harddup
in the section
Improving muxing and A/V sync reliability
or in the manual page.

7.8.2.3. Sample Rate Conversion

If the audio sample rate in the original file is not the same as
required by the target format, sample rate conversion is required.
This is achieved using the -srate option and
the -af lavcresample audio filter together.

DVD:

-srate 48000 -af lavcresample=48000

VCD and SVCD:

-srate 44100 -af lavcresample=44100

7.8.3. Using libavcodec for VCD/SVCD/DVD Encoding

7.8.3.1. Introduction

libavcodec can be used to
create VCD/SVCD/DVD compliant video by using the appropriate options.

7.8.3.2. lavcopts

This is a list of fields in -lavcopts that you may
be required to change in order to make a complaint movie for VCD, SVCD,
or DVD:

acodec:
mp2 for VCD, SVCD, or PAL DVD;
ac3 is most commonly used for DVD.
PCM audio may also be used for DVD, but this is mostly a big waste of
space.
Note that MP3 audio is not compliant for any of these formats, but
players often have no problem playing it anyway.

abitrate:
224 for VCD; up to 384 for SVCD; up to 1536 for DVD, but commonly
used values range from 192 kbps for stereo to 384 kbps for 5.1 channel
sound.

vcodec:
mpeg1video for VCD;
mpeg2video for SVCD;
mpeg2video is usually used for DVD but you may also use
mpeg1video for CIF resolutions.

keyint:
Used to set the GOP size.
18 for 30fps material, or 15 for 25/24 fps material.
Commercial producers seem to prefer keyframe intervals of 12.
It is possible to make this much larger and still retain compatibility
with most players.
A keyint of 25 should never cause any problems.

vrc_buf_size:
327 for VCD, 917 for SVCD, and 1835 for DVD.

vrc_minrate:
1152, for VCD. May be left alone for SVCD and DVD.

vrc_maxrate:
1152 for VCD; 2500 for SVCD; 9800 for DVD.
For SVCD and DVD, you might wish to use lower values depending on your
own personal preferences and requirements.

vbitrate:
1152 for VCD;
up to 2500 for SVCD;
up to 9800 for DVD.
For the latter two formats, vbitrate should be set based on personal
preference.
For instance, if you insist on fitting 20 or so hours on a DVD, you
could use vbitrate=400.
The resulting video quality would probably be quite bad.
If you are trying to squeeze out the maximum possible quality on a DVD,
use vbitrate=9800, but be warned that this could constrain you to less
than an hour of video on a single-layer DVD.

vstrict:
vstrict=0 should be used to create DVDs.
Without this option, MEncoder creates a
stream that cannot be correctly decoded by some standalone DVD
players.

7.8.3.4. Advanced Options

For higher quality encoding, you may also wish to add quality-enhancing
options to lavcopts, such as trell,
mbd=2, and others.
Note that qpel and v4mv, while often
useful with MPEG-4, are not usable with MPEG-1 or MPEG-2.
Also, if you are trying to make a very high quality DVD encode, it may
be useful to add dc=10 to lavcopts.
Doing so may help reduce the appearance of blocks in flat-colored areas.
Putting it all together, this is an example of a set of lavcopts for a
higher quality DVD:

7.8.4. Encoding Audio

VCD and SVCD support MPEG-1 layer II audio, using one of
toolame,
twolame,
or libavcodec's MP2 encoder.
The libavcodec MP2 is far from being as good as the other two libraries,
however it should always be available to use.
VCD only supports constant bitrate audio (CBR) whereas SVCD supports
variable bitrate (VBR), too.
Be careful when using VBR because some bad standalone players might not
support it too well.